This invention relates generally to traction vehicles such as locomotives that have thermal prime movers on board, and it relates more particularly to means for automatically detecting the presence of a locomotive inside a tunnel.
Large self-propelled traction vehicles such as locomotives commonly use a thermal prime mover to drive an electrical transmission comprising generating means for supplying electric current to a plurality of direct current (d-c) traction motors whose rotors are drivingly coupled through speed-reducing gearing to the respective axle-wheel sets of the vehicle. The generating means typically comprises a main 3-phase traction alternator whose rotor is mechanically coupled to the output shaft of the prime mover (typically a 16-cylinder turbocharged diesel engine). When excitation current is supplied to field windings on the rotating rotor, alternating voltages are generated in the 3-phase stator windings of the alternator. These voltages are rectified and applied to the armature windings of the traction motors.
During the "motoring" or propulsion mode of operation, a locomotive diesel engine tends to deliver constant power, depending on throttle setting and ambient conditions, regardless of locomotive speed. For maximum performance, the electrical power output of the traction alternator must be suitably controlled so that the locomotive utilizes full engine power. For proper train handling, intermediate power output levels are provided to permit graduation from minimum to full output. But the load on the engine must not exceed whatever level of power the engine can develop. Overloads can cause premature wear, engine stalling or "bogging," or other undesirable effects. Historically, locomotive control systems have been designed so that the operator can select the desired level of traction power in discrete steps between zero and maximum, and so that the engine develops whatever level of power the traction and auxiliary loads demand.
Engine horsepower is proportional to the product of the angular velocity at which the crankshaft turns and the torque opposing such motion. For the purpose of varying and regulating the amount of available power, it is common practice to equip a locomotive engine with a speed regulating governor which adjusts the quantity of pressurized diesel fuel (i.e., fuel oil) injected into each of the engine cylinders so that the actual speed (RPM) of the crankshaft corresponds to a desired speed. The desired speed is set, within permissible limits, by a manually operated lever or handle of a throttle that can be selectively moved in eight steps or "notches" between a low power position (N1) and a maximum power position (N8). The throttle handle is part of the control console located in the operator's cab of the locomotive. (In addition to the eight conventional power notches, the handle has an "idle" position and a "shutdown" position).
The position of the throttle handle determines the engine speed setting of the associated governor. In a typical governor system, the output piston of an electro-hydraulic device is drivingly connected, via a mechanical linkage, to a pair of movable fuel pump racks which in turn are coupled to a plurality of fuel injection pumps that respectively meter the amounts of fuel supplied to the power cylinders of the engine. The governor compares the desired speed (as commanded by the throttle) with the actual speed of the engine, and its output piston moves the fuel racks as necessary to minimize any deviation therebetween.
For each of its eight different speed settings, the engine is capable of developing a corresponding constant amount of horsepower (assuming maximum output torque). When the throttle notch 8 is selected, maximum speed (e.g., 1,050 rpm) and maximum rated gross horsepower (e.g., 4,000) are realized. Under normal conditions the engine power at each notch equals the power demanded by the electric propulsion system which is supplied by the engine-driven main alternator plus power consumed by certain electrically and mechanically driven auxiliary equipments.
The output power (KVA) of the main alternator is proportional to the product of the rms magnitudes of generated voltage and load current. The voltage magnitude varies with the rotational speed of the engine, and it is also a function of the magnitude of excitation current in the alternator field windings. For the purpose of accurately controlling and regulating the amount of power supplied to the electric load circuit, it is common practice to adjust the field strength of the traction alternator to compensate for load changes and minimize the error between actual and desired KVA. The desired power depends on the specific speed setting of the engine. Such excitation control will establish a balanced steady-state condition which results in a substantially constant, optimum electrical power output for each position of the throttle handle.
In practice the above-summarized system of controlling a diesel-electric locomotive also includes suitable means for overriding normal operation of the system and reducing engine load in response to certain temporary abnormal conditions, such as loss of wheel adhesion, low pressure in the lubricating oil system or the engine coolant system, or a load exceeding the power capability of the engine at whatever speed the throttle is commanding. This response, which is generally referred to as "deration," helps the locomotive recover from such conditions and/or prevents serious damage to the engine. In addition, the excitation control system conventionally includes means for limiting or reducing alternator output voltage as necessary to keep the magnitude of this voltage and the magnitude of load current from respectively exceeding predetermined safe maximum levels or limits. Current limit is effective when the locomotive is accelerating from rest. At low locomotive speeds, the traction motor armatures are rotating slowly, so their back emf is low. A low alternator voltage can now produce maximum load current which in turn produces the high tractive effort required for acceleration. On the other hand, the alternator voltage magnitude must be held constant at its maximum level whenever locomotive speed is high. At high speeds the traction motor armatures are rotating rapidly and have a high back emf, and the alternator voltage must then be high to produce the required load current.
One of the conditions that require deration of the locomotive propulsion system is excessive engine heat. If for any reason the engine cooling system were unable to function effectively and the engine were to become overheated, the locomotive control system responds to the resulting rise of temperature by reducing the traction load on the engine. In a typical prior art system this deration occurs when the temperature of the engine lubricating oil exceeds a predetermined overtemperature threshold (e.g., 235.degree. F.), and it will reduce the load to a predetermined fraction (e.g., about 3/4) of normal for the selected throttle position. If the engine were even hotter, as indicated by either the temperature of the coolant exceeding the aforesaid threshold for a predetermined length of time or by the oil temperature attaining a higher level (e.g., 245.degree. F.), all traction load is dropped and the engine is automatically returned to its idle speed regardless of the throttle position.
The above-summarized prior art temperature-responsive deration can be caused by unusually high ambient air temperature, even though there is no malfunction of the engine or its cooling system. Hot air reduces the engine-cooling effectiveness of the water-to-air heat exchangers (radiators) in the cooling system. Such a condition can occur when a locomotive is traversing a tunnel that is relatively long and not well ventilated. In a consist of two or more locomotives, the last one of the trail unit(s) is most likely to overheat because it passes through confined space preheated by the exhaust gases of each preceding unit. The resulting deration of one or more locomotives in a tunnel will adversely affect the performance of the derated locomotive(s), thereby undesirably increasing the running time and decreasing the productivity of the consist. Such deration is not really necessary so long as the engine and its various support systems are functioning properly and so long as the locomotive is not inside the tunnel too long. Ordinarily a locomotive propulsion system and its components have short-time ratings that appreciably exceed their ratings for continuous duty, and therefore the locomotive can safely endure overheating caused by a tunnel environment for at least a short term (e.g., under approximately ten minutes). Accordingly, when a locomotive engine is temporarily overheated because the locomotive is passing through a tunnel, it is both desirable and feasible to cancel or delay the normal temperature-responsive deration that is provided in prior art locomotive propulsion systems.